Switch matrices are commonly used in the field of electronics to route electrical signals. FIG. 1 depicts a prior art switch matrix 100. The switch matrix 100 includes a plurality of overlapping microstrips 102a–102p arranged to form a grid. The microstrips are conductive lines fabricated on a semiconductor substrate 104, which is typically GaAs to minimize signal losses. At their cross points, i.e., the points where horizontal microstrips 102a–h cross vertical microstrips 102i–p, the microstrips are electrically isolated from one another. A switch (represented by the diode symbol in FIG. 1), e.g., switch 106, is present at each cross point to couple the microstrips that intersect at that cross point.
Each switch can either be open (i.e., prevent an electrical connection from being made through the switch) or closed (i.e., permit an electrical connection to be made through the switch). By using a controller 110 to control which of the switches are open and which of the switches are closed, the switch matrix 100 can be used to route electrical signals. For example, if an input electrical signal is present on horizontal microstrip 102a and only the switch 106 in the upper left hand corner of the switch matrix 100 is closed, the input electrical signal is passed to vertical microstrip 102i. If only the switch 108 in the upper right hand corner of the switch matrix 100 is closed, the input electrical signal is passed to vertical microstrip 102p. Closing of multiple switches will allow the input electrical signal to be passed to multiple microstrips.
A known switch commonly used in prior art switch matrices is a PNPN latching device. The PNPN latching device is coupled between a horizontal microstrip and a vertical microstrip and includes a biasing port for receiving a typical biasing current. The PNPN latching device is closed by applying a large differential voltage across the device. For example, if switch 106 is a PNPN latching device, switch 106 may be closed by supplying a positive voltage, e.g., greater than 10 V, to horizontal microstrip 102a and a negative voltage, e.g., less than −10 V, to vertical microstrip 102i. Once the PNPN latching device is closed, the PNPN latching device will remain closed as long as a relatively small biasing current is passing through the PNPN latching device. The PNPN latching device may be opened by removing this biasing current, thereby uncoupling microstrips 102a and 102i. This switching action is due to the inherent hysteresis of the PNPN device, which is well known to those skilled in the art.
Switch matrices of this type have four significant limitations. First, when high frequency signals, e.g., in the Gigahertz range, are passing through one or more of the microstrips 102a–102p, excessive coupling occurs between the microstrips at the cross points. For example, in a typical switch matrix using 25 micrometer wide microstrips over a 100 micrometer deep GaAs substrate, isolation can be less than −20 dB at 25 Gigahertz. Attempting to decrease the coupling by reducing the width of the transmission lines, results in increased insertion losses. Second, the GaAs substrates that are typically used in switch matrices to minimize signal losses are relatively expensive as compared to silicon substrates. Third, the PNPN latching devices that are commonly used are difficult to design and produce, leading to relatively high fabrication costs. Finally, due to size and yield restrictions, it is difficult to incorporate amplifiers into the switch matrix to amplify the RF signal.
Accordingly, there is a need for switch matrices with improved isolation that are easier to design and inexpensive to produce. The present invention fulfills this need among others.